1. Field of the Invention
The present invention relates to non-impact printing apparatus and methods for use in recording and more particularly to those employing light-emitting diode recording elements.
2. Brief Description of the Prior Art
In the prior art as exemplified by U.S. Pat. No. 4,746,941 a light-emitting diode (LED) printhead may include several thousand LED's arranged in a row. Typically, these LED's will be formed on individual LED chip arrays carrying say 64, 96 or 128 LED's and thirty or so of these chip arrays will be positioned end-to-end to form the string of LED's. Electrical drive current is fed to each LED using integrated circuit driver chips. Two of the driver chips, one on each side of the LED chip array will provide drive current to the LED's of that array. In order to control the timing and enablement of the drive currents, various timing and data signals are provided to the driver chips. In the case of a grey level printhead, a multibit data signal is used to determine the duration of a recording period for recording a pixel (picture element) by an individual LED. In a typical grey level printhead six data bits carried by six data lines may be used to define the duration of a recording period. In addition, power and other timing signals are required to be provided to each driver chip. Thus, perhaps each driver may require twenty input pads for receiving these different signals where power inputs are doubled to carry the increased current. With two driver chips used to drive the odd and even numbered LED's of an LED chip array there may be required 40 or more electrical connections to test the operability of an LED chip array and its two driver chips. In such a test, current is provided to the LED's at an appropriate duty cycle and the unit tested for say 1000 hours of "on" time. If a duty cycle of 50% is employed, the test period should take 2000 hours. The test is referred to as a burn-in test and relies upon the characteristic of LED's that when new, their intensities are brightest but this brightness tends to decay with age and reaches an equilibrium at about for example 1000 hours of age for some LED's. Thus, to provide a printhead that will be reasonably uniform with age, it is conventional to "burn-in" the LED's until they reach this equilibrium level.
It is known in the art to prepare the LED's and driver chips in a module so that the module may be tested and then tested modules are assembled onto a support to form the printhead. Where three LED chip arrays are provided on each module along with six driver chips, a testable module could require more than 120 electrical connections. Accessing that number of points is not practical, in addition the bond pads on the driver chips risk being damaged in the testing process where they would be engaged by signal probes.
In U.S. application Ser. No. 07/455,125, filed Dec. 22, 1989 in the names of Beaman et al, a spreader circuit board is described for distributing signals to each of three driver chips on the module. This board connects all of the driver input lines together into one common set of larger input bond pads. With two spreader boards on a module, the number of input pads to each spreader board can be reduced to say 20. This number is still difficult to be used even with precision wafer probe test fixtures connected to both sides of a typical assembly.
The making of electrical connections to these input pads is not a problem during normal printhead assembly using standard wire bonding practices. The problems manifest during initial power testing, burn-in and life testing. In initial power-up, testing could be accomplished using two sets of contacts, typically an elastomeric or wafer probe needles. These suffer from alignment mismatch, high contact resistance, limited lifetime of the contacts, complex fixturing and slow loading times. Major problems arise during elevated temperature burn-in. The previously described contact methods are typically not suitable for use in hostile environments when a highly reliable connection is required to conduct high-speed logic signals and high currents over an extended length of time, say 100 to 1000 hours.
It is, therefore, an object of the invention to provide a means for providing fewer test pad connections for burn-in testing of the recording elements of a printhead. With fewer connections, there is more available area to provide larger contact pads to more practically accommodate signal probe test pins with improved tolerance for misalignment of the pins in the testing fixture.